1. Field of the Invention
The present invention relates to a process for producing thin film magnetic heads. More particularly the invention relates to a process for producing a thin film magnetic head for use in writing data on or reading data from magnetic recording media such as magnetic tapes or magnetic discs, the head being suited especially to high-density recording.
2. Description of the Prior Art
Thin film magnetic heads have the features of being small-sized, assuring facilitated recording of data with a high density in a multiplicity of channels and being adapted to form tracks of more accurate and reduced width. They are small in inductance, low in core loss and therefore excellent in radio-frequency characteristics, and have the ability for recording with high resolution because they produce a sharp magnetic field distribution. In addition to these features in characteristics, these magnetic heads are amenable to mass production at a greatly reduced cost. With the requirement for high-density magnetic recording in recent years, thin film magnetic heads have proved remarkably superior because of these features, and active research and development efforts have been made in this field (U.S. Pat. No. 4,677,036, etc.).
The main construction of such a thin film magnetic head will be described with reference to FIG. 3.
A substrate 1 has formed thereon a lower magnetic core layer 2. The substrate 1 is made of a material having high wear resistance, such as ferrite, alumina or glass. The lower magnetic core layer 2 is in the form of a film of a soft magnetic metal such as Ni-Fe, Fe-Al-Si or Co-based amorphous alloys and provides one of a pair of cores of the magnetic head. An upper magnetic core layer 3 is formed over the lower core layer 2. Like the lower layer 2, the upper core layer 3 is made of a soft magnetic metal film and serves as the other core of the magnetic head. Provided between the upper core layer 3 and the lower core layer 2 is an insulating layer 4 forming a thin magnetic gap 5 at the tip of the head and having an electrically conductive coil layer 6 embedded therein. The conductive coil 6 is in the form of a spiral coil extending continuously rightward in FIG. 3 and is shown in section of its one side. The two core layers 2, 3 are joined together at the portion C illustrated. The conductive coil layer 6 is made of a film of a conductor such as Cu, Al, Au or Ag. The insulating layer 4 is made of a film of a nonmagnetic material such as SiO.sub.2, Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4.
The thin film magnetic head is fabricated by forming over the substrate 1, the lower magnetic core layer 2 and the insulating layer 4 having a specified shape and embedding the conductive coil 6 as shown in FIG. 3 (b). Thereafter, the upper magnetic core layer 3 is formed in a vapor phase in a predetermined pattern over an area .alpha. to cover the magnetic gap 5 and the joint portion C (FIG. 3 (c)). Further the tip of the resulting assembly over a predetermined length .beta. is removed by grinding.
The upper magnetic core layer 3 is formed conventionally in the predetermined pattern, by the wet etching method, ion milling method or lift-off method.
When the wet etching method is resorted to, the upper core layer 3 is masked with a photoresist film and then etched in the predetermined pattern with an etching solution. This method has an advantage in that the layer can be etched in a short period of time without the necessity of using an expensive apparatus. The ion etching method is a physical etching method wherein the impact of Ar ion or a like inactive ion is used instead of the etching solution.
The lift-off method forms the upper magnetic core layer 3 in the predetermined pattern without etching.
The lift-off method will be described briefly with reference to FIGS. 4(a and b). To schematically illustrate this method FIGS. 4(a and b) show a case wherein the upper magnetic core layer 3 is formed directly on a substrate.
A photoresist film 12 is first formed on the substrate and then etched in a specified pattern, which is reverse to the pattern of the core layer 3 to be formed. Subsequently, an upper magnetic material layer 13 is formed over the entire upper surface obtained, to later provide the upper magnetic core layer 3. As seen in FIG. 4(a), therefore, the portion of the magnetic material layer 13 corresponding to the pattern of the upper core layer 3 to be formed, is formed directly on the substrate 11, with the other unnecessary portion thereof formed on the photoresist film 12. The substrate 11 is then immersed in its entirety in acetone or a like organic solvent, whereupon the organic solvent penetrates through the clearances (indicated by arrows A shown) between the magnetic material layer 13 on the substrate 11 and the magnetic material layer 13 on the photoresist film 12, to dissolve the photoresist film 12. With the dissolution of the photoresist film 12, the magnetic material layer 13 over the film 12 is separated off. The result is that the remaining portion of the upper magnetic material layer 13 provides the upper magnetic core layer 3 in the predetermined pattern as shown in FIG. 4(b).
With a trend in recent years toward higher-density magnetic recording, recording media with a high coercive force have been placed into use. For use with the recording media of high coercive force, there arises a need for thin film magnetic heads having sufficient ability to magnetize these media. Accordingly, in addition to the use of materials having a highly saturation magnetic flux density for the magnetic core layers 2, 3 of recent thin film magnetic heads, the upper magnetic core layer 3, which was about 1 to about 5 .mu.m in thickness, is now made as thick as about 10 to about 30 .mu.m and thereby prevented from magnetic saturation.
Nevertheless, the foregoing known methods of forming the upper magnetic core layer 3 encounter the following problems when the layer has such a large thickness.
(1) When such a thick core layer 3 is to be patterned by the wet etching method, the amount of side etching increases. This presents difficulties in controlling the width of the core layer 3 with a high accuracy. Consequently, it is impossible to reduce the track width to about 10 to about 20 .mu.m as required in recent years, and to determine the width with a tolerance of .+-.1 to 2 .mu.m.
(2) The ion milling method does not permit selective etching for the upper magnetic core layer 3 because of its nature as a physical etching method. Further, the photo-resist film, serving as a mask, and the insulating layer 4, are always etched with the core layer 3.
Accordingly, when a thick upper magnetic core layer 3 is to be etched, there arises a need to form a photoresist film of larger thickness. This is difficult to form. Even if it is possible to form the thicker photoresist film, extreme difficulties are encountered in patterning the film with high accuracy and in forming the core layer 3 with high accuracy.
Further in the actual etching step, the core layer 3 is slightly overetched so as to cancel any influence by variations in the thickness in the layer 3 and in the etching rate from portion to portion. The degree of overetching poses no problem when the core layer is several micrometers in thickness, but produces an unnegligible influence if the thickness is as large as 10 to 30 .mu.m. More specifically, it is likely that the etching is continued even after the core layer 3 has been etched through the entire thickness thereof, possibly removing the insulating layer 4. This results in objections such as exposure of or damage to the conductive coil layer 6 and a break in this layer 6.
(3) The lift-off method is free of the above drawbacks since the upper magnetic material layer 13 is not etched in this method. However, it is required that the photoresist film formed be eventually removable by dissolving with an organic solvent, whereas when subjected to a high temperature, the photoresist film is liable to change into a film which is sparingly soluble with the organic solvent. Accordingly, if the photoresist film formed is heated to a high temperature (for example, of not lower than 150.degree. C.), the film becomes unremovable or less-removable by dissolving in the final step.
When the magnetic material layer is to be formed in a vapor phase, therefore, the substrate temperature must be lower than usually (e.g. lower than 100.degree. C.). However, the lower substrate temperature reduces the adhesion of the magnetic core layer to the underlying layer and entails impaired magnetic characteristics.
Furthermore, some kinds of photoresist films as formed are inherently sparingly soluble with a conventional organic solvent and are thus not suitable for the lift-off method.
The present invention has been accomplished in view of the foregoing situation, especially to overcome the problems involved in the fabrication of thin film magnetic heads utilizing the lift-off method.
Incidentally, proposals have already been made in the field of production of semiconductors to use plasma etching in place of the organic solvent used for the lift-off method for removing the photoresist film (Unexamined Japanese Patent Publication No. SHO 52-100983; J. Vac. Sci. Technol., 19(4), pp. 1324-1328, Nov./Dec. 1981). Nevertheless, nothing whatsoever is known as to the application thereof to the fabrication of thin film magnetic heads or the expected effect thereof.